Alkoxysilanes are useful as starting materials for various silane coupling agents, insulating thin films, etc In particular, trialkoxysilanes are in large demand as having an SiH bond in the molecule thereof and being chemically more stable than monoalkoxysilanes or dialkoxysilanes, and it has been demanded to develop a process for producing them at high efficiency and low cost.
Known processes for producing a trialkoxysilane include a process comprising reacting a chlorosilane with a lower alkyl alcohol. However, besides being expensive, the chlorosilane by-produces hydrochloric acid, which not only makes purification of the product difficult but causes corrosion of the reaction apparatus.
A process comprising directly reacting metallic silicon and an alkyl alcohol is also known. This reaction is carried out in a vapor phase of a liquid phase in the presence of a catalyst, e.g., a copper catalyst, as described in U.S. Pat. Nos. 3,072,700 and 4,931,578. Where the reaction is effected in a liquid phase, the reaction rate and the silicon conversion per unit reactor volume are lower and the cost is higher, due to use of reaction solvents having a high boiling point, than in the reaction in a vapor phase. In addition, since the reaction residue contains high-boiling products, etc. and is therefore sludgy, it is difficult to treat. Further, the reaction solvent may be incorporated into the reaction product, making purification of the product complicated. In order to increase selectivity of the trialkoxysilane, it is known to add a halide to the liquid reaction system, as described in U.S. Pat. No. 4,931,578. However, the halide added is dissolved in a solvent used in large quantities so that it must be used in large quantities. As a result, the halide and the solvent react with each other to produce a by-product, which makes the solvent non-reusable.
A vapor phase process for efficiently producing a trialkoxysilane has thus been desired. A conventionally known vapor phase process as described in J. Amer. Chem. Soc., Vol. 70, p. 2170 (1945) preferentially produces a tetraalkoxysilane, a higher order compound, resulting in a low selectivity of the desired trialkoxysilane. Besides, the process achieves a low silicon conversion.
In addition, the inventors have found in the course of their study that the conventional vapor phase process involves the following problem when carried out on an industrial scale by using a fixed-bed reactor. Because the direct reaction between metallic silicon and an alkyl alcohol is a solid-gas reaction accompanied by great heat generation, it is influenced by cooling conditions when effected on an increased scale. In some cases, it is necessary to drop the feed rate of the alcohol in order to control the reaction, resulting in a reduction in reaction rate. Further, a temperature distribution occurs in the reactor, making it difficult to control the temperature uniformly. As a result, a tetraalkoxysilane and a dialkoxysilane are by-produced besides the desired trialkoxysilane, resulting in a substantial reduction in yield of the trialkoxysilane.
Where a fluidized bed reactor is used for uniform reaction in the reactor and for easy control of the reaction temperature, and the reaction is carried out at a lower temperature in order to increase the selectivity of a trialkoxysilane, there would arise various problems, such as a cessation of the fluid state, a reduction in conversion of metallic silicon, an increase of by-products, and the like, consequently failing to increase the selectivity as expected.